Today marks the 1st of Muḥarram, which means it is the beginning of the new Islamic year.
The start of the Islamic calendar was set by ʿUmar ibn al-Khaṭṭāb رضي الله عنه, who based the first year when the Prophet ﷺ migrated (made Hijrah) from Makkah to Madīnah.
It is now 1448 years since the Prophet ﷺ's migration to Madīnah, however the date of the Hijrah is not linked with the 1st of Muḥarram.
Al-Muḥarram [ٱلْمُحَرَّم] means forbidden, it is one of the months when fighting is forbidden.
Narrated Abū Hurayrah رضي الله عنه: The Prophet ﷺ said,
“The most excellent fast after Ramaḍān is Allāh's month, al-Muḥarram, and the most excellent prayer after what is prescribed is prayer during the night.”
[Ṣaḥīḥ Muslim 1163a]